One-pot hydrothermal synthesis of magnetic La-based composites for phosphate removal and recovery: effect of Lanthanum forms and adsorption mechanism

Magnetic La-based composites with different La forms—Fe 3 O 4 /La(OH) 3 (MLH), Fe 3 O 4 /LaCO 3 OH (MLCH), and Fe 3 O 4 /La 2 (CO 3 ) 3 (MLC)—were synthesized via a one-pot hydrothermal method at 90°C. This low-temperature process utilizes nitrate ions as oxidants, yielding diverse La forms in a single step while reducing energy input and process complexity. The effect of La forms on phosphate adsorption was systematically evaluated. MLH exhibited the highest adsorption capacity (106.30   mg/g) with nearly stoichiometric P/La utilization (0.993), whereas MLCH attained the fastest kinetics. Both MLH and MLCH maintained high performance over a wide pH range (4.0–9.0), while MLC performed optimally under mildly acidic conditions (4.0–6.0) but was adversely affected by bicarbonate ions. All composites demonstrated excellent magnetic recovery and reusability, with capacity retention above 94.9% after five cycles. Furthermore, phosphate recovery, achieved via adsorption–desorption cycles followed by CaCl 2 -induced precipitation, confirmed the dual utility of these materials for both pollutant removal and resource recovery. In real wastewater treatment, either 0.05   g/L of MLH or 0.1   g/L of MLCH reduced the phosphate concentration from 2.25   mg/L to 0.021   mg/L. While the adsorption mechanism for all composites involves electrostatic attraction and ligand exchange, the specific La form markedly influences overall performance. This study provides valuable insight into the role of La forms in phosphate adsorption and introduces a simple, low-energy synthesis approach for preparing high-performance magnetic La-based adsorbents.